Models of Succession

Clements (1916) noted that comparison of successional stages is less useful than is understanding of processes affecting the transitions from one sere to another. Nevertheless, few studies have continued over sufficient periods to evaluate the mechanism(s) producing successional transitions. Rather, a number of nonmutually-exclusive models, all of which may affect particular transitions to varying degrees, have been proposed and debated widely (e.g., Connell and Slatyer 1977, H. Horn 1981, Mcintosh 1981, Peet and Christensen 1980). The debate involves competing views of succession as (1) resulting from population dynamics or emergent ecosystem processes and (2) as stochastic or converging on an equilibrial community structure (H. Horn 1981, Mcintosh 1981).

The facilitation model was proposed by Clements (1916), who viewed communities as an entity that showed progressive (facilitated) development similar to the ontogeny of individual organisms.According to this model, also called relay floristics (Egler 1954), successive stages cause progressive changes in environmental conditions that facilitate their replacement by the subsequent stage, and later successional species cannot appear until sufficient environmental modification by earlier stages has occurred. For example, soil development or increased plant density during early stages makes the environment less suitable for recruitment of additional early, r-selected species but more suitable for recruitment of later, K-selected species. Fire-dominated ecosystems (in which nitrogen is volatilized during fire) usually are colonized following fire by symbiotic nitrogen fixers such as alders, Alnus spp., ceanothus, Ceanothus spp., or cherries, Prunus spp. These species are relatively shade intolerant, and increasing density eventually suppresses their photosynthesis and nitrogen fixation, facilitating replacement by shade-tolerant species growing in the understory and exploiting the replenished organic nitrogen in the soil (e.g., Boring et al. 1988). The increasing porosity and altered nutrient content of decomposing wood, resulting from het-erotroph activity, precludes further recruitment of early successional species (e.g., bark beetles and anaerobic or microaerophilic microorganisms), and facilitates replacement by later successional wood borers and more aerobic microorganisms (e.g., Schowalter et al. 1992).

This model was challenged early. Gleason (1917,1926,1927),Whittaker (1953, 1970), and more recently Drury and Nisbet (1973) argued for a reductionist view of species colonization and turnover on the basis of individual life history attributes. Connell and Slatyer (1977), H. Horn (1981), and MacMahon (1981) proposed a broader view of succession as reflecting multiple pathways and mechanisms.

Egler (1954) argued that secondary succession often may reflect differential longevity of colonizing species. Most of the eventual dominants colonize early when competition is low. Failure of species to become established at this early stage reduces the probability of future dominance. Juveniles of later species grow to maturity over a longer period, tolerating the early dominance of ruderal species, and eventually exclude the early successional species (e.g., through shading, preemptive use of water, etc.). Connell and Slatyer (1977) referred to this model as the tolerance model. This model is represented best in ecosystems dominated by species that sprout from roots or stumps, germinate from seed banks, or colonize rapidly from adjacent sources. These attributes ensure early appearance along with more ruderal species. However, many large-seeded trees, flightless arthropods, and other animals characterizing later successional stages of forest ecosystems require a long period of establishment and achieve dominance only during late succession, especially in large areas of disturbed habitat (e.g., Shure and Phillips 1991).

A third model proposed by Connell and Slatyer (1977) to explain at least some successional transitions is the antithesis of facilitation. According to this inhibition model, the initial colonists preempt use of resources and exclude, suppress, or inhibit subsequent colonists for as long as these initial colonists persist. Succession can proceed only as individuals are damaged or killed and thereby release resources (including growing space) for other species. Examples of inhibition are successional stages dominated by allelopathic species, such as shrubs that increase soil salinity or acidity; by species that preempt space, such as many perennial sodforming grasses whose network of rhizomes restrict establishment by other plants; by species whose life spans coincide with the average interval between disturbances; and by species that create a positive feedback between disturbance and regeneration, such as eucalypts, Eucalyptus spp. (e.g., Shugart et al. 1981). In decomposing wood, the sequence of colonization by various insects determines initial fungal association; initial colonization by mold fungi can catab-olize available labile carbohydrates and inhibit subsequent establishment by decay fungi (Kaarik 1974), restricting further succession. Environmental fluctuation, disturbances, or animal activity (such as gopher mounds, bison wallows, trampling, and insect outbreaks) often are necessary to facilitate replacement of these stages (MacMahon 1981, Schowalter et al. 1981a, Schowalter and Lowman 1999). However, Agee (1993), Schowalter (1985), and Schowalter et al. (1981a) noted that bark beetle outbreaks increase fuel accumulation and the probability of fire, thereby ensuring the continuity of pine forest (Fig. 10.5).

H. Horn (1981) developed a model of forest succession as a tree-by-tree replacement process using the number of saplings of various species growing under each canopy species (ignoring species for which this is not a reasonable predictor of replacement) and correcting for expected longevity. This model assumes that knowing what species occupies a given position narrows the statistical range of expected future occupants and that the probability of replacement

Tolerance Model Succession

| Diagrammatic representation of interactions between southern pine beetle, Dendroctonus frontalis, and fire in the southeastern coniferous forest. Successional transitions extend from left to right; dotted arrows indicate direction of movement. Fire is a regular feature of the generally dry uplands but moves into generally moist lowlands where drought or southern pine beetle creates favorable conditions for combustion. Southern pine beetle is a regular feature of both forests but is most abundant where pines occur at high density and stress levels. Fire is necessary for regeneration of pines, especially following succession to hardwoods if fire return is delayed. Schowalter et al. (1981a) with permission from the Entomological Society of America.

| Diagrammatic representation of interactions between southern pine beetle, Dendroctonus frontalis, and fire in the southeastern coniferous forest. Successional transitions extend from left to right; dotted arrows indicate direction of movement. Fire is a regular feature of the generally dry uplands but moves into generally moist lowlands where drought or southern pine beetle creates favorable conditions for combustion. Southern pine beetle is a regular feature of both forests but is most abundant where pines occur at high density and stress levels. Fire is necessary for regeneration of pines, especially following succession to hardwoods if fire return is delayed. Schowalter et al. (1981a) with permission from the Entomological Society of America.

depends only on the species occupying that position and does not change with time unless the occupant of that position changes. The model is not directly applicable to communities in which recurrent large-scale disturbances are the primary factor affecting vegetation dynamics. It is interesting that H. Horn (1981) found that successive iterations by a given replacement matrix invariably converged on a particular community composition, regardless of the starting composition. This result indicates that convergence is not necessarily a reflection of biotic processes (Horn 1981) and should increase attention to the rate of convergence and transition states producing convergence. E. Evans (1988) reported that grasshopper assemblage structure in replicate plots in a grassland ecosystem converged (i.e., became significantly more similar than predicted by a random model) during recovery from fire (Fig. 10.6).

Many ecologists consider vegetation changes over time to be no more than expressions of species life history characteristics. Species distributions in time reflect their physiological tolerances to changing environmental conditions,

Relay Floristics Model

NUMBER OF CONSECUTIVE MOVES

Displacement of individual grasshopper communities (A with, and B without, the unusually common Phoetaliotes nebrascensis) from initial ordination positions after 1-4 "moves" (1-4 years), as observed on study sites at the Konza Prairie Long Term Ecological Research Site in Kansas, United States, 1982-1986, and as predicted by the correlated random walk model. Vertical lines represent 95% confidence limits. From E. Evans (1988) with permission from Oikos. Please see extended permission list pg 571.

parallel to distributions in space (Botkin 1981, Drury and Nisbet 1973). Several major simulation models of forest gap succession are based on species-specific growth rates and longevities as affected by stochastic mortality (e.g., Doyle 1981, Shugart et al. 1981, Solomon et al. 1981). Platt and Connell (2003) explored effects of relationships between early colonists and later colonists on species replacement following catastrophic versus noncatastrophic disturbances as explanation for variable successional trajectories, depending on disturbance severity and relative survival of early and late successional species. However, Blatt et al. (2001) presented the only model that currently addresses the contribution of animals to the successional process. The variety of successional pathways determined by unique combinations of interacting initial and subsequent conditions may favor models that apply chaos theory.

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